This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. In our recently completed intervention study commencing in 1999, we have been examining the mechanisms of physical activity behavior by tracking adolescent females for a full school year in a controlled exercise intervention. This intervention was unique in several ways: 1) it targeted sedentary, unfit adolescent females;2) it tracked not only behavior and fitness over time but also likely psychosocial mediators of behavior change (e.g., self-efficacy, social support, perceived barriers to activity, etc?);and 3) it incorporated both psychosocial and physiological hypotheses in an integrated study with a physiological endpoint. Results demonstrate that the intervention was successful in terms of enhancing physical fitness and physical activity levels among sedentary adolescent females;yet, none of the proposed psychosocial mediators assessed were able to explain the behavior change, and the impact of the intervention on participants'fitness (i.e., VO2max, percent body fat) was minimal.